There are three similar units of mass called the ton or tonne:

  • tonne (SI) or metric ton = 1000 kg (2204 lb). The official symbol is t, but mT and T are also used.
  • short ton (or simply ton in casual use in U.S.) = 907.185 kg (2000 lb)
  • long ton (or weight ton) = 1016.047 kg (2240 lb), commonly used in the UK and for petroleum products.

The word ton or tonne is derived from the Old English tunne, and ultimately from the Old French tonne, and referred originally to a large cask with a capacity of 252 gallons, which holds approximately 2016 pounds of water. Such a barrel is still called a tun in British English, but usage is now dying out.

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The freight ton or measurement ton is also used as what is technically a measure of volume when measuring ship capacities (tonnage) or freights. One measurement ton is equal to:
  • 40 cubic feet
  • 1.48148 cubic yards
  • 1,132.67 litres
  • 1.13267 cubic metres
The amount of water displaced by a volume of one measurement ton has a mass similar to the tons of mass listed above, 1,132.7 kg or 2497.1 lb. The measurement ton is abbreviated as M/T, MT or MTON which can cause it to be confused with the metric ton.

The register ton is also a unit of volume, defined as 100 cubic feet. It is often abbreviated as GRT for gross registered ton.

See 1 E-1 m for a comparison with other volumes, also orders of magnitude (volume).

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